U.S. Pat. No. 4,238,812 discloses an overcurrent protection device that includes a positive temperature coefficient (PTC) element having two opposite surfaces, two electrodes respectively connected to the surfaces of the PTC element, and two pins respectively connected to the electrodes. The positive temperature coefficient element is made of a polymeric material containing a conductive filler.
Typically, the pins of the overcurrent protection device described above are usually solid circular wires that are made of tinned copper (9.5 wt % of tin and 90.5 wt % of copper, having a thermal conductivity of 369.27 W/mK) and that each has a cross-sectional area of 0.205 mm2 (24 AWG, diameter of 0.511 mm).
The ratio of the trip current to the work current of the overcurrent protection device is usually set in the range of 2.0±0.2. However, the overcurrent protection device with the ratio of 2.0±0.2 would not be suitable for various apparatus if circuits of the apparatus were to be destroyed upon the trip current reaching two times the work current.